The proposal investigates the mechanisms underlying the accurate production of limb movements and force changes directed towards a target. Four groups of experiments are planned and explore hypotheses derived from pilot data. First, we will determine the features of trajectories which remain invariant when responses are directed to targets at different distances. Pilot data suggests that such responses are regulated in duration and controlled only in amplitude. We will now explore the generality of this control policy by determining how it is affected by changes in initial conditions, loads, accuracy and context. Second, we will determine how the control policy is implemented. Pilot data is compatible with the view that control is achieved through a push-pull mechanism utilizing agonist and antagonist. The antagonist is proposed to be used to linearize the properties of the peripheral plant. The mechanical contributions of each will be determined. Third, we will explore the central mechanisms responsible for specifying response direction and amplitude through a new paradigm. Pilot data indicates that target stimuli is processed by two independent channels. One appears to trigger a response conforming to a pre-computed estimate. The second updates this estimate from stimulus information. Fourth, the functional deficits underlying cerebellar dysmetria will be analyzed to ascertain the abnormalities in scaling, updating and correction.